The invention relates to a fuel nozzle and more particularly to an end cap plate of a xe2x80x9cDual Fuelxe2x80x9d nozzle design that has been configured for gas only use and to an adaptation for cooling the same.
Gas turbines for power generation are generally available with fuel nozzles configured for either xe2x80x9cDual Fuelxe2x80x9d or xe2x80x9cGas Onlyxe2x80x9d. xe2x80x9cGas Onlyxe2x80x9d refers to operation burning, for example, natural gas and xe2x80x9cDual Fuelxe2x80x9d refers to having the capability of operation burning either natural gas or liquid fuel. The dual fuel configuration is generally applied with oil used as a backup fuel, if natural gas is unavailable. The gas only configuration is offered in order to reduce costs as the nozzle parts and all associated equipment required for liquid fuel operation are not supplied. In general, fuel nozzles are designed to have dual fuel capability and the gas only version is a modification to the dual fuel design in which the dual fuel parts, which include the oil, atomizing air and water passages, are removed from the nozzle. The removal of these components exposes a cylindrical, open region along the axial center line of the nozzle to hot combustion gas. An example of a dual fuel nozzle modified to remove the dual (liquid) fuel parts is illustrated in FIG. 1. This nozzle is disclosed in detail in copending application Ser. No. 09/021,081, filed Feb. 10, 1998, the entire disclosure of which is incorporated herein by this reference.
FIG. 1 is a cross-section through the burner assembly. The burner assembly is divided into four regions by function including an inlet flow conditioner 7, an air swirler assembly with natural gas fuel injection (referred to as a nozzle assembly) 2, an annular fuel air mixing passage 3, and a central diffusion flame natural gas fuel swozzle assembly 13.
Air enters the burner from a high pressure plenum 5, which surrounds the entire assembly except the discharge end, which enters the combustor reaction zone 6. Most of the air for combustion enters the premixer via the inlet flow conditioner (IFC) 7. The IFC includes an annular flow passage 8 that is bounded by a solid cylindrical inner wall 9 at the inside diameter, a perforated cylindrical outer wall 10 at the outside diameter, and a perforated end cap 11 at the upstream end. In the center of the flow passage 8 is one or more annular turning vanes 12. Premixer air enters the IFC 7 via the perforations in the end cap 11 and the cylindrical outer wall 10.
At the center of the burner assembly is a conventional diffusion flame fuel nozzle tip 13 having a slotted gas tip 14, which receives combustion air from an annular passage 15 and natural gas fuel through gas holes 16. The body of this fuel nozzle includes a bellows 17 to compensate for differential thermal expansions between this nozzle and the premixer. In the center of this diffusion flame fuel nozzle is a cavity 18, which, as noted above, receives the liquid fuel assembly to provide dual fuel capability. In the dual fuel configuration, during gas fuel operation, the oil, atomizing air and water passages in this region are purged with cool air to block hot gas from entering the passages when not in use. When the nozzle is configured for gas only operation, cavity 18 must be capped at the distal end of the nozzle to block hot combustion gas from entering the center, open region which may result in mechanical damage due to the high temperature. Since the end cap plate is exposed to hot combustion gas, it must be cooled.
In the past, cooling of the end cap plate used to cover the open region at the nozzle tip in a conversion from a dual fuel to a gas only configuration has been accomplished using the gas fuel as the cooling medium. More specifically, because removal of the dual fuel components eliminates the structure that formed the inner wall of the gas fuel passage, a part of the gas fuel can effuse through tiny holes in the end cap plate (not shown in FIG. 1) to cool the same while the bulk of the fuel passes through the normal gas hole injectors 16 which are located between the air swirler vanes. This is a very simplified design for converting from a dual fuel to gas only nozzle. While generally effective, this approach is undesirable in view of the need to maintain low emissions over the gas turbine operating range. Diverting gas fuel for cooling from the desired injection points between the air swirler vanes and injecting that gas at a different location through tiny holes in an end cap plate (not shown in FIG. 1) for cooling reduces the premixing of gas fuel and air which is essential for low emissions performance.
Another possible method for cooling the end cap plate is to use the cooling air supplied from the nozzle purge air system. The nozzle purge air system supplies air cooled so that its temperature does not exceed 750xc2x0 F. As briefly described above with reference to purging the liquid fuel components during gas fuel operation, this air is generally applied to purging the gas fuel passages when not in use to resist the back-flow of hot combustion gas into the gas passages, manifolds and pipings. The limit of not exceeding an air temperature of 750xc2x0 F. relates to the possible auto-ignition of gas fuel coming into contact with air exceeding that temperature. Since an end cap plate passage adapted to receive purge air for cooling rather than gas fuel would never have gas fuel present, it would be inefficient to use specially cooled air from the nozzle purge system to cool an end cap plate.
The existing fuel nozzle purge system does not have the capacity to supply the additional amount of air required for cooling the gas only nozzle end cap plate, nor would such a use of that specially cooled air be efficient.
It has been determined, however, that compressor discharge air would be an adequate cooling medium. Thus, a diffusion flame nozzle gas tip has been designed to allow for the use of compressor discharge air to cool the end cap plate. The appropriate amount of compressor discharge air is extracted from annular passage 15 into the central region 18 and is emitted through tiny (effusion) holes in the end cap plate to produce the desired cooling.
Thus, the invention is embodied in a method for cooling the end cap plate of a gas only fuel nozzle in which compressor discharge air is supplied as the cooling medium. The method of the invention advantageously replaces the requirement to use either cooling air from the existing nozzle purge system or gas fuel as the cooling medium. In accordance with an embodiment of the invention, this is accomplished by providing a diffusion flame nozzle gas tip that diverts compressor discharge air from the passage feeding the diffusion nozzle air swirl vanes to the cavity vacated by removal of the dual fuel components so that the diverted compressor discharge air can flow to and through effusion holes in the end cap plate. In a preferred embodiment, the nozzle gas tip defines a cavity for receiving the compressor discharge air from a peripheral passage of the nozzle for flow through the effusion openings defined in the end cap tip.